I doubt that headrests were intentionally made to be window smashers. However, it might be worth a try to use one of the metal prongs for that task. It would be unwieldy, but it might work. A sharp impact strike in a small area will easily break out a car side window (nor the windshield though).

In all my years as a paramedic, I never saw glass broken out by a head rest. By heads, yes, but head rests, no.